Object reference not set to an instance of an object : Minecraft modpack

Im trying to start a server with my friends on the modpack Roguelike adventures and dungeons. And i Installed it Perfectly fine and i got the server running but whenever my friends try to download any version of the modpack it has the error message "Object reference not set to an instance of an object" ive tried getting them the mods other ways and it doesnt work. Is there anything we can do?